1. Field of the Invention
The present disclosure relates to an organic light emitting diode display having high aperture ratio and a method for manufacturing the same. Especially, the present disclosure relates to a bottom emission type organic light emitting diode display and a method for manufacturing the same, in which the four electrodes are overlapped each other for forming the storage capacitor ensuring more storage capacitor with minimized plane area.
2. Discussion of the Related Art
Nowadays, various flat panel display devices are developed for overcoming many drawbacks of the cathode ray tube such as heavy weight and bulk volume. The flat panel display devices include the liquid crystal display device (or LCD), the field emission display (or FED), the plasma display panel (or PDP) and the electroluminescence device (or EL).
The electroluminescence display device is categorized in the inorganic light emitting diode display device and the organic light emitting diode display device according to the luminescence material. As a self-emitting display device, the electroluminescence display device has the merits those the response speed is very fast, the brightness is very high and the view angle is large.
FIG. 1 is a diagram illustrating the structure of the organic light emitting diode. As shown in FIG. 1, the organic light emitting diode comprises the organic light emitting material layer, and the cathode and the anode which are facing each other with the organic light emitting material layer therebetween. The organic light emitting material layer comprises the hole injection layer HIL, the hole transport layer HTL, the emission layer EML, the electron transport layer ETL and the electron injection layer EIL. The organic light emitting diode radiates the lights due to the energy from the excition formed at the excitation state in which the hole and the electron are recombined at the emission layer EML.
The organic light emitting diode radiates the lights due to the energy from the excition formed at the excitation state in which the hole from the anode and the electron from the cathode are recombined at the emission layer EML. The organic light emitting diode display can represent the video data by controlling the amount (or ‘brightness’) of the light generated and radiated from the emission layer ELM of the organic light emitting diode as shown in FIG. 1.
The organic light emitting diode display (or OLED) using the organic light emitting diode can be categorized in the passive matrix type organic light emitting diode display (or PMOLED) and the active matrix type organic light emitting diode display (or AMOLED).
The active matrix type organic light emitting diode display (or AMOLED) shows the video data by controlling the current applying to the organic light emitting diode using the thin film transistor (or TFT).
FIG. 2 is the exemplary circuit diagram illustrating the structure of one pixel in the active matrix organic light emitting diode display (or AMOLED). FIG. 3 is a plane view illustrating the structure of one pixel in the AMOLED. FIG. 4 is a cross sectional view along the cutting line I-I′ for illustrating the structure of the AMOLED.
Referring to FIGS. 2 and 3, the active matrix organic light emitting diode display comprises a switching thin film transistor ST, a driving thin film transistor DT connected to the switching thin film transistor ST, and an organic light emitting diode OLED connected to the driving thin film transistor DT.
The switching thin film transistor ST is formed where the scan line SL and the data line DL is crossing. The switching thin film transistor ST acts for selecting the pixel which is connected to the switching thin film transistor ST. The switching thin film transistor ST includes a gate electrode SG branching from the gate line GL, a semiconductor channel layer SA overlapping with the gate electrode SG, a source electrode SS and a drain electrode SD. The driving thin film transistor DT acts for driving an anode electrode ANO of the organic light emitting diode OD disposed at the pixel selected by the switching thin film transistor ST. The driving thin film transistor DT includes a gate electrode DG connected to the drain electrode SD of the switching thin film transistor ST, a semiconductor channel layer DA, a source electrode DS connected to the driving current line VDD, and a drain electrode DD. The drain electrode DD of the driving thin film transistor DT is connected to the anode electrode ANO of the organic light emitting diode OLED.
Referring to FIG. 4 more detail, on the substrate SUB of the active matrix organic light emitting diode display, the gate electrodes SG and DG of the switching thin film transistor ST and the driving thin film transistor DT, respectively are formed. On the gate electrodes SG and DG, the gate insulator GI is deposited. On the gate insulator GI overlapping with the gate electrodes SG and DG, the semiconductor layers SA and DA are formed, respectively. On the semiconductor layer SA and DA, the source electrode SS and DS and the drain electrode SD and DD facing and separating from each other are formed. The drain electrode SD of the switching thin film transistor ST is connected to the gate electrode DG of the driving thin film transistor DT via the contact hole penetrating the gate insulator GI. The passivation layer PAS is deposited on the substrate SUB having the switching thin film transistor ST and the driving thin film transistor DT.
Especially, in the case that the semiconductor layers SA and DA include the oxide semiconductor material, thanks to the characteristics of high electron mobility, it has merit for applied to a large area thin film transistor substrate having large charging capacitor. However, in order to ensure the stability of the oxide semiconductor material, it is preferable to include an etch stopper SE and DE covering the upper surface of channel area to protect them from etchants. In detail, the etch stoppers SE and DE would be formed to protect the semiconductor layers SA and DA from being back-etched by the etchant for patterning the source electrodes SS and DS and the drain electrodes SD and DD.
A color filer is formed at the area where the anode electrode ANO will be formed later. It is preferable for the color filter CF to have a large area as possible. For example, it is preferable to overlap with some portions of the data line DL, the driving current line VDD and/or the scan line SL. The upper surface of the substrate having these thin film transistors ST and DT and color filters CF is not in even and/or smooth conditions, but in uneven and/or rugged conditions having many steps. In order that the organic light emitting diode display has good luminescent quality over the whole display area, the organic light emitting layer OLE should be formed on an even or smooth surface. So, to make the upper surface in planar and even conditions, the over coat layer OC is deposited on the whole surface of the substrate OC.
Then, on the over coat layer OC, the anode electrode ANO of the organic light emitting diode OLED is formed. Here, the anode electrode ANO is connected to the drain electrode DD of the driving thin film transistor DT through the contact hole penetrating the over coat layer OC and the passivation layer PAS.
On the substrate SUB having the anode electrode ANO, a bank BANK is formed over the area having the switching thin film transistor ST, the driving thin film transistor DT and the various lines DL, SL and VDD, for defining the light emitting area. The exposed portion of the anode electrode ANO by the bank BANK would be the light emitting area. On the anode electrode ANO exposed from the bank BANK, the organic light emitting layer OLE is formed. On the organic light emitting layer OLE, the cathode electrode ACT is formed.
For the case that the organic light emitting layer OLE has a material emitting the white lights, each pixel can represent various colors by the color filter CF disposed under the anode electrode ANO. The organic light emitting diode display as shown in FIG. 4 is the bottom emission type display in which the visible light is radiated to the bottom direction of the display substrate.
In the bottom emission type organic light emitting diode display, the storage capacitor STG is formed where the gate electrode DG of the driving thin film transistor DT and the anode electrode ANO are overlapped. For the organic light emitting diode display, it can represent the video data by driving the organic light emitting diode. In general, the required energy for driving the organic light emitting diode is higher than any other electric element used for representing video data.
In order to exactly and quickly represent the moving video data of which electric data is very quickly varied, the large amount of the storage capacitor is required for the organic light emitting diode display. For ensuring enough storage capacitor, the surface area of the storage capacitor should be large. For the case of the top emission type organic light emitting diode display, as the storage capacitor can be formed by overlapping with the emission area, it can ensure enough large area of the storage capacitor without reduction of the aperture ratio. On the contrary, for the case of the bottom emission type organic light emitting diode display, as the area of the storage capacitor is getting larger, the area for representing light i.e., the aperture ratio would be reduced. That is, in the bottom emission type organic light emitting diode display, the area of the storage capacitor directly affects to the aperture ratio reduction.